Increases in the capacity of hard disk drives has been demanded more in recent years. Further, with advancements incorporating hard disk drives in household electric appliances, the demands for miniaturization and capacity increases of hard disk drives is increasing. A hard disk drive using a longitudinal recording system has achieved a real recording density exceeding 20 gigabit per 1 cm2. However, it has become difficult to further increase recording density by using this system. Therefore, a perpendicular recording system is now used instead of the longitudinal recording system. The influence of a diamagnetic field in a high density recording area is relatively small in the perpendicular recording system as compared with the longitudinal recording system, and it is thought that the perpendicular recording system is advantageous in increasing recording density.
In a perpendicular magnetic recording medium for use in the perpendicular recording system, a recording layer comprising a CoCrPt alloy conventionally used in longitudinal recording media, has been utilized. However, for the purpose of further reduction of a noise, a granular-type recording layer comprising a CoCrPt alloy with oxygen or oxide is added, is proposed. The granular-type recording layers are disclosed in Japanese Patent Publication Nos. 2001-222809 and 2003-178413. In the case of conventional recording layers comprising a CoCrPt alloy, noise is reduced by segregation of a nonmagnetic material mainly comprising Cr into a grain boundary. The segregation is achieved by making use of phase separation of Co and Cr, which causes magnetic isolation of the magnetic crystal grains. For heightening the noise reduction, addition of Cr in a large amount is helpful, but adding large amount of Cr causes a large amount of Cr to be in the magnetic crystal grains. Large amounts Cr in the magnetic crystal grain can cause magnetic anisotropy energy to lower and the stability of recording signals to deteriorate. However, in the case of a granular-type recording layer of a CoCrPt alloy to which oxygen or oxide is added, a media can be formed without adding a large amount of Cr. Since the oxide is easily separated from the magnetic crystal grains, if a template that is a cue to form a grain boundary of oxide has been formed as the under layer, then a structure where the oxide surrounds magnetic crystal grains can be formed. Since the amount of Cr contained in magnetic crystal grains can be reduced, it is possible to reduce noise without lowering magnetic anisotropy energy.
In the perpendicular recording system, recording and reproducing efficiencies in the perpendicular direction can be heightened by the combination of a magnetic recording medium having a soft magnetic layer between the recording layer and the substrate with a single magnetic pole head. The single magnetic pole head can be a magnetic head having a structure with a shield on the trailing side of the main magnetic pole (a TS head), or a magnetic head having a structure with a shield surrounding the main magnetic pole (a WAS head). These head types improve the recording magnetic field gradient. Although recording magnetic field gradient is improved with a magnetic head provided with such a shield, the magnetic field strength is lowered at the same time. Therefore, sufficient overwriting characteristics may be difficult to obtain with a medium in which intergranular interaction in the granular recording layer is sufficiently reduced for the purpose of noise reduction. Thus, the improvement of overwriting characteristics is obtained by forming a thick non-granular structural ferromagnetic layer on a granular recording layer (see Japanese Patent Publication Nos. 2004-310910 and 2006-351055).
Further, lowering of a noise by the improvement of a granular recording layer is also considered. As an example of such consideration, it is proposed to use two or more kinds of oxides to form the grain boundary of a granular recording layer (Japanese Patent Publication Nos. 2005-100537 and 2006-164440).
A granular-type recording layer comprising a CoCrPt alloy to which oxygen or an oxide is added, is good in grain boundary formation, and therefore reduces noise. Further, sufficient overwriting characteristics can be obtained by the formation of a non-granular structural recording layer on a granular structural recording layer even when a magnetic head provided with a shield is used. However, further improvement is required for achieving higher density recording. Moreover, since the decrease of magnetic field strength in recording by narrowing of a track of a magnetic head is possible, the distance between the magnetic head and a soft magnetic layer is shortened in order to compensate for the decrease of magnetic field strength. In that case, if an intermediate layer is thinned more than necessary, the crystallographic orientation of a recording layer is deteriorated and it becomes difficult to form a template structure that is a cue to form a grain boundary of oxide. Accordingly, thinning of a recording layer and a protective layer is also important.